Generally, conventional white lighting apparatuses including a light-emitting diode (LED) or a laser diode (LD) as a light source provide white light by means of a blue LED or LD in combination with a phosphor emitting light of yellow color (i.e., a complementary color of blue).
Hitherto, the matrix or material of the phosphor (or a fluorescent body) employed in such lighting apparatuses is a resin material, a glass material, or a ceramic material. In a trend for using a high-output light source in recent years, the phosphor is worked at higher temperature. Thus, ceramic phosphors having high durability have been developed.
For example, such a ceramic phosphor is generally formed of a garnet-type ceramic material represented by formula A3B5O12. Among such ceramic phosphors, yttrium aluminum garnet (YAG: Y3Al5O12) emits yellow fluorescence when the elemental cerium (Ce) is used as an activator.
The below-described prior art documents (Patent Documents 1 to 3) disclose a ceramic composite body exhibiting improved properties (e.g., durability), the ceramic composite body containing alumina (Al2O3) having high thermal conductivity in which YAG:Ce or Lu3Al5O12:Ce (LuAG:Ce) is dispersed and deposited.
Specifically, in Patent Documents 1 to 3, the balance between color unevenness and thermal conductivity is controlled on the basis of the volume ratio of Al2O3/A3B5O12:Ce.
For example, in Patent Document 1, the YAG:Ce content of the entire composite body is adjusted to 22 to 55 vol. %. According to Patent Document 1, a YAG:Ce content less than 22 vol. % leads to an increase in the amount of blue light transmission and occurrence of color unevenness, whereas a YAG:Ce content more than 55 vol. % leads to a decrease in thermal conductivity and a reduction in durability.
In Patent Document 2, the LuAG:Ce content is adjusted to 25 to 95 vol. %. In Patent Document 3, the A3B5O12:Ce content is adjusted to 20 to 25 vol. %. According to Patent Document 3, CeAl11O18 (i.e., a Ce source) is used in an amount of 0.5 to 5 vol. % for reducing the volatilization of Ce during a firing process.